How Do You Calculate Displacement and Acceleration in a Kinematics Car Problem?

[miamiheat5]

A car starts from rest heading due east. It first accelerates at 3.0m/s^2 for 5.0s and then continues without further acceleration for 20.0s. It then brakes for 8.0s in coming to rest. (a) What is the car's velocity after the first 5.0 seconds? (b) What is the car's acceleration over the last 8.0s interval? (c) What is the total displacement?

The Attempt at a Solution

This is what i got: a) vf=vi+at
=0(3)(5)
vf=15m/s

b) vf=vi+at
0=15+a(8)
-15=a(8)
a=-1.7m/s^2

c) d1=37.5m
d2=300m
d3=65.6m
dtot=403.1m

 

[neutrino]

This is what i got: a) vf=vi+at
=0(3)(5)
vf=15m/s

Correct.

b) vf=vi+at
0=15+a(8)
-15=a(8)
a=-1.7m/s^2

15/8 is not 1.7

c) d1=37.5m
d2=300m
d3=65.6m
dtot=403.1m

d3 would change based on the answer to b.

 

[miamiheat5]

ya it should be -1.9